The present invention relates to a semiconductor device and a process for producing the same. More particularly, the present invention relates to an improvement of a high electron mobility transistor, hereinafter referred to as a HEMT, and an improved process for producing a HEMT.
The HEMT is an active semiconductor device and its operating principle is completely distinguishable from that of silicon ICs and the GaAs FETICs.
The HEMT has a first single crystalline semiconductor layer, a second single crystalline semiconductor layer having an electron affinity different from that of the first single crystalline semiconductor layer and forming a heterojunction with respect to the first single crystalline semiconductor layer, and a gate electrode on the second single crystalline semiconductor layer. An electron-storing layer is formed in proximity to the heterojunction due to the difference in the electron affinity and is used as the conduction channel of the HEMT. The electron mobility of the HEMT is approximately 10 times that of GaAs FETICs and approximately 100 times that of silicon ICs, at the temperature of liquid nitrogen (77K). The electron-storing layer mentioned above contains a quasi two-dimensional electron gas, the electrons of which gas being predominent current-conduction carriers of the HEMT. In addition, the sheet-electron concentration of the quasi two-dimensional electron gas is controlled by applying voltage through the gate electrode. The impedance of the conduction channel is controlled by a pair of electrodes located beside the gate electrode. A very high electron mobility of the HEMT at a low temperature, for example at 77K, is due to the fact that the electron mobility of the quasi two-dimensional electron gas is very high at such a temperature where impurity scattering predominantly controls the electron mobility. The quasi two-dimensional electron gas is formed in proximity to the heterojunction, as stated above, and the thickness of the quasi two-dimensional electron gas is approximately the amount of spreading electron waves or on the order of ten angstroms. The relationship between the quasi two-dimensional electron gas and the single crystalline semiconductor layers which supply electrons to the electron storing layer is a spatially separated one, with the result that the electron mobility of the quasi two-dimensional electron gas is not lessened due to the reduction of impurities in one single crystalline semiconductor layer. As a result, it is possible to achieve a very high electron mobility at a low temperature where the electron mobility is predominantly controlled by impurity scattering.
There are two types of operation of the HEMT, namely the normally on-type (depletion (D) type) of operation and the normally off-type (enhancement (E) type) of operation. The layer structure and the thickness of the first and second single crystalline semiconductor layers determine which type of operation the HEMT has. More specifically, in a case where the electron affinity of the second single crystalline semiconductor layer is greater than that of the first single crystalline semiconductor layer, the normally on-type operation occurs when the metallurgical thickness of the second single crystalline semiconductor layer (upper layer) is greater than a certain critical amount which depends on the aforementioned three parameters and the properties of the gate which is to be formed on the upper layer. The normally off-type operation occurs when the metallurgical thickness of the upper layer is smaller than the certain critical amount mentioned above. Similarly, in a case where the second (upper) and first (lower) single crystalline layers have a small and great electron affinity, respectively, the normally on-type operation occurs when the metallurgical thickness of the lower layer is greater than a certain critical amount, which depends on the layer parameters. The normally off-type operation occurs when the metallurgical thickness of the lower layer is smaller than the certain critical amount. The metallurgical thickness is hereinafter simply referred to as the thickness.
The prior art is explained more in detail with reference to FIGS. 1 through 3.
Referring to FIG. 1, Ec.sub.1 indicates the energy of the electrons in the second single crystalline semiconductor layer having a low electron affinity, for example, an N-doped AlGaAs (aluminum gallium arsenide) layer, and Ec.sub.2 indicates the energy of the electrons in the first single crystalline semiconductor layer having a high electron affinity, for example, an undoped GaAs layer. Because of the difference in the electron affinity, a substantial part of the electrons in the N-doped AlGaAs layer are attracted to the undoped GaAs layer and quasi two-dimensional electron gas (2DEG) is formed as shown in FIG. 1 while the positively ionized impurities (+) remain in the N-doped AlGaAs layer. The symbol "E.sub.F " indicates the Fermi level, and the symbol "Ecg" indicates an energy gap, at the interface of the heterojunction between the N-doped AlGaAs layer and the undoped GaAs layer. The two semiconductors capable of forming the heterojunction of the HEMT should have a sufficient difference in electron affinity, as was explained above. Furthermore, since the region near the interface of the heterojunction should be free of any crystal defects, the lattice constants of the two semiconductors should be close to one another. In addition, in order to attain a satisfactorily high energy gap (Ecg) at the interface of the heterojunction, the difference between the energy gap in the two semiconductors should be great. The types of semiconductors which satisfy the above-described three properties, i.e. electron affinity, lattice constant, and energy gap, are numerous as shown in Table 1.
TABLE 1 ______________________________________ Lattice Electron Band Gap Constants Affinity Nos. Semiconductors (eV) (.ANG.) (eV) ______________________________________ 1 Aluminum gallium 1.8 5.657 3.77 arsenide (Al.sub.0.3 Ga.sub.0.7 As) Gallium arsenide 1.43 5.654 4.07 2 Aluminum gallium 1.8 5.657 3.77 arsenide (Al.sub.0.3 Ga.sub.0.7 As) Germanium 0.66 5.658 4.13 3 Gallium arsenide 1.43 5.654 4.07 Germanium 0.66 5.658 4.13 4 Cadmium telluride 1.44 6.477 4.28 Indium antimonide 0.17 6.479 4.59 5 Gallium antimonide 0.68 6.095 4.06 Indium arsenide 0.36 6.058 4.9 ______________________________________
Referring to FIG. 2, a conventional normally off-type HEMT is shown. Reference numeral 1 indicates a substrate made of single crystalline semi-insulating semiconductor material. Reference numeral 2 indicates the undoped GaAs layer 2, i.e. the first single crystalline semiconductor layer having a great electron affinity. On the undoped GaAs layer 2, the N-doped AlGaAs layer 3 and a gate electrode 6 are successively formed. The aluminum gallium arsenide of the N-doped AlGaAs layer 3 is the main composition of the second single crystalline semiconductor layer and has a small electron affinity, and the gate electrode 6 controls the concentration of the quasi two-dimensional electron gas 5 in the electron-storing layer 18. The electrons of the quasi two-dimensional electron gas 5 are supplied from the N-doped AlGaAs layer 3 into the undoped GaAs layer 2 when a predetermined voltage is applied via the gate electrode 6 to the heterojunction at an energized state. The N.sup.+ -doped regions are selectively formed on the undoped GaAs layer 2 in a selfalignment manner with respect to the N-doped AlGaAs layer 3. These N.sup.+ -doped regions are the source region 4A and drain region 4B, and the distance between the source region 4A and the drain region 4B corresponds to the gate length. The source electrode 7A and the drain electrode 7B control the impedance of the conduction channel or the impedance of the undoped GaAs layer 2 in proximity to the interface 8 of the heterojunction.
The HEMT has an advantage of high electron mobility, as stated above, and therefore is able to provide super high-speed logic elements. When the gate length is shortened, the high-speed feature of the HEMT becomes more outstanding due to the shortened traveling time of the electrons. However, when the gate length is considerably shortened, the distance between the source region 4A and the drain region 4B may be so short that a punch-through phenomenon occurs, resulting in electrons in the source region 4A being injected into the undoped GaAs layer 2. When the punch-through phenomenon occurs, the threshold voltage of the HEMT is disadvantageously decreased.
In the HEMT, the quasi-dimensional electron gas 5, the electons of which gas being the predominant current-conduction carriers in the HEMT, is generated due to the difference in electron affinity, and the normally off-type operation is achieved when the thickness of the N-doped AlGaAs layer 3 (the semiconductor having a small electron affinity) is smaller than that of the undoped GaAs layer 2 (the semiconductor having a great electron affinity) by a certain critical value. Therefore, the normally off-type operation can also be achieved in the structure shown in FIGS. 2 and 3. The N-doped AlGaAs layer 3 shown in FIG. 2 has been selectively removed so that the quasi two-dimensional electron gas 5 is generated upon the application of voltage from the gate electrode 6 exclusively beneath the N-doped AlGaAs layer 3. Contrary to this, in a normally off-type HEMT such as shown in FIG. 3, the N-doped AlGaAs layer 3 is not selectively removed but is formed entirely on the undoped GaAs layer 2 while the gate electrode 6 is embedded in the N-doped AlGaAs layer. In addition, no quasi two-dimensional electron gas 5 is formed in the conduction channel beneath the gate electrode 6 by means of selectively decreasing the thickness of the N-doped AlGaAs layer 3.
The quasi two-dimensional electron gas 5 is formed in the undoped GaAs layer 2 is proximity to the interface 8 of the heterojunction between the undoped GaAs layer 2 and the N-doped AlGaAs layer 3 on which the gate electrode 6 is not located. The source region 4A and drain region 4B are not positioned in a self-alignment relationship with respect to the gate electrode 6, but they are sufficiently separated from the conduction channel 9. In the normally off-type HEMT shown in FIG. 3, it is not necessary to shorten the distance between the source region 4A and the drain region 4B, but, it is only necessary to decrease the length of the gate electrode 6 so that the traveling time of the electrons across the conduction channel 9 is shortened. It would be quite obvious to an expert having an ordinary knowledge of HEMTs as to how to modify the normally off-type HEMT shown in FIG. 2 so as to devise the one shown in FIG. 3. Nevertheless, the normally off-type HEMT shown in FIG. 3 is not realistic for the following reasons.
The most important thing in the production of HEMT's is to stably form or not form a quasi two-dimensional electron gas 5 (FIGS. 2 and 3) and 2 DEG (FIG. 1) in the conduction channel 9. In order to stably form or not form a quasi two-dimensional electron gas in the conduction channel 9, not only the selection of the above-described three properties, i.e. electron affinity, lattice constant and energy gap, but also strict control of the thickness of the undoped GaAs layer 2 and the N-doped AlGaAs layer 3 must be carried out. Furthermore, the interface 8 of the heterojunction must be provided with such a structure that the composition abruptly changes from, for example, AlGaAs to GaAs at the interface 8. The type of technique of crystal growth by which strict control of the thickness of the undoped GaAs layer 2 and the N-doped AlGaAs layer 3 and an abrupt change in the composition can now be achieved is the molecular beam epitaxy (MBE) method. With this method the crystal growth rate can be strictly controlled so that one atom layer epitaxially grows per second. Therefore, the crystal growth of the undoped GaAs layer 2 and the N-doped AlGaAs layer 3 is carried out by means of the MBE method. In addition, the N-doped AlGaAs layer 3 (FIG. 2) must be selectively removed with such accuracy that the quasi two-dimensional electron gas 5 is not formed in the conduction channel 9 in the normal state. However, the accuracy of conventional etching methods cannot exceed the order of 100 angstroms and are not at all sufficient for stably attaining the normally off-type operation or for precisely controlling the threshold voltage of a HEMT.